Lucas Hamilton is Manager, Building Science Applications
Buildings have no idea what the air temperature is outside. To our buildings and their components, the outer world is their outer surface. Surface temperature is influenced by many things in addition to air temperature. One of the most dramatic influences on surface temperature is radiation and this swings wildly in the course of a day.
In the past, we’ve talked about how for the most part windows are energy pigs. Well, they are energy pigs because windows have really poor R values (resistance to conductivity), often leak too much air after a few years of use, and transmit too much infrared radiation when we don’t want them to. When you talk about energy flow with regard to conductivity, the rate at which energy flows is basically the difference in surface temperature across the assembly – not air to air temperature across the assembly – multiplied by its conductivity. If it’s really cold outside and you want to have less energy flowing through your R4 windows in terms of conductivity make the outside surface of the window hot. That will decrease the delta -T and slow down the conductivity.
There are a variety of ways that would help to make windows hot in the winter and cool in the summer. Some are passive solar technologies that we re-learned in the 1970’s and re-forgot in the 1980’s. When the sun is high in the sky it is summer and you want the shade to cool off the surface of the window and the shade devise to insulate it from incident infrared. In the winter, the sun is low on the horizon and you want to capture that infrared and heat up the surface of the window. That will slow down the conductivity through it by decreasing the delta -T. If we can’t make R15 windows – which we cannot practically or affordably with our current technologies – then we have to find a way to trick the windows into thinking that it is warmer in the winter and cooler in the summer.
Again, it’s all about surface temperature. A building doesn’t know what the air temperature is – it only knows what its surface temperature is. We need to use that knowledge to be a little bit smarter and to stop fighting nature and instead work with it.